The invention discloses a special nodulizer for low-temperature impact resistant and high-ductility nodular cast iron and a method for preparing the special nodulizer. Components of the special nodulizer include, in mass percent, 0.5-1.5% of RE, 4-7% of Mg, 42-49% of Si, 0.5-3% of Ca, less than or equal to 0.5% of Al, less than 0.4% of MgO and the balance Fe. The special nodulizer requires high purity, has the stable components, and is small in segregation, strict in granular control and reasonable in distribution. When in use, the special nodulizer has the advantages that nodularity is higher than 85% and recession resistance is high when Mg residues in castings are controlled within the low limit of 0.036-0.040%, Mg absorption rate is high, addition of the special nodulizer is decreased by 20-30% as compared with that of a general nodulizer, less slag is generated in nodulizing reaction, slag inclusion defects of the castings are obviously decreased, magnetic particle inspection passing rate is obviously increased, the number of graphite nodules ranges from 120 to 150, the tendency of shrinkage porosity of the castings is obviously decreased, and ultrasonic inspection passing rate is obviously increased. The content of rare earth in a traditional common nodulizer generally ranges from 2% to 6%, the content of rare earth in the special nodulizer ranges from 0.5% to 1.5%, and previous rare earth resources are saved.